1. Field of the Invention
The present invention relates to a nonwoven fabric made of an ethylene/tetrafluoroethylene copolymer.
2. Discussion of Background
Nonwoven fabrics are used as fiber materials suitable for reinforcing filters, electrolyte membranes, is etc.
As polymer materials for the nonwoven fabric, general-purpose materials such as polypropylene resins, polyester resins or polyamide resins have been mainly used. However, a fluororesin which is excellent in heat resistance, chemical resistance, non-stickiness and cleanness has been proposed as a material for an air-cleaning ultra high performance filter or a chemical filter in the field of semiconductors, a filter bag for an antipollution measures, etc.
There is a method wherein a polytetrafluoroethylene (PTFE) resin as such a fluororesin, is stretched, cut and formed into fibers, followed by a water jet method or a needle punching method to form a nonwoven fabric. However, the strength of such a nonwoven fabric is insufficient, since fibers are not mutually fused.
On the other hand, in a case where a nonwoven fabric product is produced by meltblowing a thermoplastic fluororesin, there are problems such that it is difficult to form the thermoplastic fluororesin into fine fibers, and the nonwoven fabric thereby obtained is poor in strength.
It is disclosed that a nonwoven fabric is produced by meltblowing an ethylene-chlorotrifluoroethylene copolymer (Patent Document 1). However, the nonwoven fabric thereby obtained is insufficient in the chemical resistance, the water repellency, the stainproofing property, the mold release characteristic, etc.
Further, it has been proposed that a nonwoven fabric is produced by meltblowing a tetrafluoroethylene copolymer having a low melt viscosity (Patent Document 2). However, since the melt viscosity of the tetrafluoroethylene copolymer is too low, the molding property is insufficient. Further, there are problems such that the handling efficiency at the time of production is insufficient, and the average fiber diameter of the nonwoven fabric thereby obtained is so small that the strength is poor. Further, if the average fiber diameter of the nonwoven fabric is increased in order to improve the strength, in a case where such a nonwoven fabric is used as a filter for removing fine particles, the maximum pore diameter becomes so large that the property for removing fine particles is poor.
On the other hand, in a case where a nonwoven fabric is produced by meltblowing an ethylene/tetrafluoroethylene copolymer which is used for generic molding, since the melt viscosity of the ethylene/tetrafluoroethylene polymer is high, the average fiber diameter of the nonwoven fabric becomes large, and if such a nonwoven fabric is used for a filter for removing fine particles, the maximum pore diameter becomes so large that the performance of removing fine particles is insufficient.
Patent Document 1: JP-A-7-229048
Patent Document 2: JP-A-2002-266219